The permeation of oxygen ions through ion transport membranes is the basis for a variety of gas separation devices and oxidation reactor systems operating at high temperatures in which permeated oxygen is recovered on the permeate side as a high purity oxygen product or is reacted on the permeate side with oxidizable compounds to form oxidized or partially oxidized products. The practical application of these gas separation devices and oxidation reactor systems requires membrane assemblies having large surface areas, means to contact feed gas with the feed sides of the membranes, and means to withdraw product gas from the permeate sides of the membranes. These membrane assemblies may comprise a large number of individual membranes arranged and assembled into modules having appropriate gas flow piping to introduce feed gas into the modules and withdraw product gas from the modules.
Ion transport membranes may be fabricated in either planar or tubular configurations. In the planar configuration, multiple flat ceramic plates are fabricated and assembled into stacks or modules having conveying means to pass feed gas over the planar membranes and to withdraw product gas from the permeate side of the planar membranes. In tubular configurations, multiple ceramic tubes may be arranged in bayonet or shell-and-tube configurations with appropriate tube sheet assemblies to isolate the feed and permeate sides of the multiple tubes.
The individual membranes used in planar or tubular module configurations typically comprise very thin layers of active membrane material supported on material having large pores or channels that allow gas flow to and from the surfaces of the active membrane layers.
The solid ion-conducting metallic oxide materials used in these membrane modules may degrade in the presence of volatile gas-phase contaminants at the high operating temperatures required to effect ion conduction, thereby reducing the ability of the membranes to conduct or permeate oxygen ions. Because of this potential problem, the successful operation of ion-conducting metallic oxide membrane systems may require control of certain contaminants in the membrane feed gas or gases.
As disclosed in U.S. Pat. No. 7,425,231, contaminants may be removed by a reactive solid material in a guard bed, the reactive solid material comprising one or more compounds selected from the group consisting of magnesium oxide, calcium oxide, copper oxide, calcium carbonate, sodium carbonate, strontium carbonate, zinc oxide, strontium oxide, and alkaline-earth-containing perovskites.
While these reactive solid materials are good getters for the contaminants, the reaction of the reactive solid materials is accompanied by expansion of the reacted solid material. For example, reaction of chromium (Cr) with magnesium oxide (MgO) forms stable bulk magnesiochromite (MgCr2O4) with an estimated 385% volume expansion of the solid phase. This may present a significant challenge for the use of guard bed materials comprising bulk reactive solid materials as disclosed in U.S. Pat. No. 7,425,231. The increase in volume could cause degradation of the guard bed and/or a reduction in void fraction accompanied by an associated increase in pressure drop.
Industry needs a contaminant removal device and process that can accommodate the volume expansion, have enough capacity to remove contaminants for years, and also have low pressure drop.
This need is addressed by embodiments of the present invention as disclosed below and defined by the claims that follow.